Invertebrates: Development of Centralized Nervous Systems

Evolution of
Nervous SystemsInvertebrates:
Development of
Centralized Nervous
Systems
Various metazoan phyla reveal a progressive
increase in complexity of nervous
systems that probably reflects in a
general way the stages in evolution of
nervous systems. The simplest pattern
of invertebrate nervous systems is the
nerve net of radiate animals, such as sea
anemones, jellyfishes, hydras, and
comb jellies (Figure 35-9A). A nerve net
is a quantum leap in complexity
beyond sensory systems of the unicellular
forms, which lack nerves. A nerve
net forms an extensive network in and
under the epidermis over all the body.
An impulse starting in one part of this
net spreads in all directions, since
synapses in most radiates do not restrict
transmission to one-way movement, as
occurs in more complex animals. There
are no differentiated sensory, motor, or
connector components in the strict
meaning of those terms. Branches of a
nerve net connect to receptors in the
epidermis and to epithelial cells that
have contractile properties, and there is
evidence of organization into reflex arcs
. Although most responses tend
to be generalized, many are astonishingly
complex for so simple a nervous
system. This type of nervous system is
found among vertebrates in nerve
plexuses located, for example, in the
intestinal wall; such nerve plexuses
govern generalized intestinal movements
such as peristalsis and segmentation.

Bilateral nervous systems, the
simplest of which occur in flatworms,
represent a distinct increase in complexity
over the nerve net of radiate
animals. Flatworms have two anterior
ganglia, composed of groups of nerve
cell bodies from which two main
nerve trunks run posteriorly, with lateral
branches extending throughout
the body (Figure 35-9B). This is the
simplest nervous system showing differentiation
into a peripheral nervous
system (a communication network
extending to all parts of the
body) and a central nervous system (a concentration of nerve cell bodies),
which coordinates everything. More
complex invertebrates exhibit a more
centralized nervous system (brain),
with two longitudinal fused nerve
cords and many ganglia. The elaborate
nervous systems of annelids contain
a bilobed brain, a double nerve
cord with segmental ganglia, and distinctive
afferent (sensory) and efferent
(motor) neurons (Figure 35-9C).
Segmental ganglia are relay stations
for coordinating regional activity.

The basic plan of molluscan nervous
systems is a series of three pairs of
well-defined ganglia, but in cephalopods
(such as octopus and squid), the
ganglia have burgeoned into textured
nervous centers of great complexity;
those of the octopus contain more than
160 million cells. Sense organs, too,
are highly developed. Consequently,
cephalopod behavior far outstrips that
of any other invertebrate.

The basic plan of arthropod nervous
systems (Figure 35-9D) resembles
that of annelids, but ganglia are larger
and sense organs are much better developed.
Social behavior is often elaborate,
particularly in hymenopteran insects
(bees, wasps, and ants), and most
arthropods are capable of considerable
manipulation of their environment.
Despite the complexity of much insect
behavior, insects are nevertheless reflexbound
animals incapable of involved
learned behavior principally because of
their small size.